Negative (-ve) loss readings - DSP FTA Series | Fluke Networks

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Negative (-ve) loss readings - DSP FTA Series

In almost every occasion, a negative loss reading is caused by an incorrect reference setting.
Check to see that the reference falls within the following ranges:- 

  • DSP-FTA410/20S
    50/125 µm Multimode Cable = -23.5 dBm to -24.5 dBm
    62.5/125 µm Multimode Cable = -19 dBm to -20.5 dBm 

  • DSP-FTA430S
    9/125 µm Singlemode Cable = -7.8 dBm to -9.5 dBm 

  • DSP-FTA440S
    50/125 µm Multimode Cable = -8.0 dBm to -10 dBm
    62.5/125 µm Multimode Cable = 8.0 dBm to -10 dBm
You can check the reference value by rotating the dial to AUTOTEST and pressing key F3. It will also give you the time the reference was made: 

DSP FTA Stored Reference Value

Figure 1. Example reference for DSP-FTA440S

If the values are too high, then try new test leads. Take a fiberscope and inspect the ends of the test leads. 

Fluke Networks Fiberscope

Figure 2. Fiberscope

As with all fiber optic test equipment, it is essential to allow the source to stabilize. As the source warms up, its output power increases. This is the primary cause of negative loss readings.
Lets take an example; below is a result with a negative loss reading. 

Cable ID: Fiber 1A                              Test Summary: PASS

FLUKE NETWORKS                                  HEADROOM: 3.07 dB (Loss)

SITE: Everett                                   Date / Time: 03/24/2002  12:43:29pam

OPERATOR: A YOUNG                               Test Standard: TIA 568B Outside FTA430

Standards Version: 4.9                          Cable Type: Singlemode

Software Version: 4.8                           FLUKE DSP-4100   S/N: 7995053 FTA430





n = 1.4677

Number of Adapters: 2

Number of Splices: 2


Propagation Delay (ns)                    5834 

Length (ft), Limit 13124                  3910  PASS


Direction                            A-B

                                1310 nm    1550 nm

Result                             PASS       PASS

Loss (dB)                         -1.05       -.69

Loss Limit (dB)                    2.69       2.69

Loss Margin (dB)                   3.74       3.38


Reference (dBm)                   -9.76      -9.32

01/24/2002  06:13:17am


Direction                            B-A

                                1310 nm    1550 nm

Result                             PASS       PASS

Loss (dB)                          -.42       -.38

Loss Limit (dB)                    2.69       2.69

Loss Margin (dB)                   3.11       3.07


Reference (dBm)                   -9.35      -9.07

03/24/2002  06:13:17am


Near End DSP-FTA S/N: 7874014

Far End DSP-FTA S/N: 7870009

Can you see what is at fault and the possible cause of the negative loss reading? Take a look at the time of the reference. Now take a look at the time of the measurement. The reference was made in the morning, before it was hot. The measurement was made over six hours later in the mid day heat. You should never allow this long a period between reference and measurement. The other factor is the temperature. The reference was made when the instrument was cool. Mid day heat will cause the source to output more power, and hence give a negative loss reading. The reference should have been made again. This is not specific to Fluke Networks equipment, it is a factor in all fiber optic test equipment. This is an extreme example.
How to stop this
Ensure you have a stable measurement. After a period of time, the power output will remain constant. The time it takes to stabilize will depend on the current conditions. If the equipment is removed from the trunk (boot) of a car in winter, it will take much more than five minutes for the source to stabilize.
How do you know when the source has stabilized? Make your reference measurement. Rotate the dial to Single Test and select Loss (Output Fiber).
You can expect the value to drift by up to 0.05 dB. When this settles and changes no more, you can be confident that the source has stabilized. If the other end of the fiber under test is some distance away, leave the remote unit on. Do remember that the source will drift by 0.25 dB over a period of eight hours.
Regular setting of the reference through out the day will ensure accurate, repeatable readings.
EN50346 Information technology - Cabling installation - Testing of installed cabling
Reference power measurements shall be repeated periodically as necessary. Situations requiring the re- establishment of reference conditions include optical power changes, temperature fluctuations, a move to a different location, and jumper/adapter replacement due to degradation.
Additional Reading
For a technical reference, probably the most comprehensive book on the subject of fiber testing is Fiber Optic Test and Measurement, edited by Dennis Derickson. Derickson assumes a bench-top environment, so he ignores some of the issues which are problems in the field, but he deals with the technical aspects. Most of it's written at a level really more suitable for the optics engineer, so we've taken the liberty of lifting information from one section for you.
In Section 9.4, Derickson discusses the uncertainties of loss measurements. He lists the causes of uncertainty in loss test, with connector variability given as the largest cause. Derickson cites uncertainties of +/- 0.3 dB with good connectors. Worn or old connectors may create larger uncertainties. All of the other measurement uncertainties (wavelength of the source, polarization mode, etc.) are much, much smaller than the connector variability; they are significant only in bench-quality measurements taken in environments where the temperature is recorded with the reading, and extreme measures have been taken to eliminate connector variability.

Learn About:
-Fiber Optics, Fiber Testing

Author: Fluke Networks

Creation Date: 2014-02-28

Last Modified: 2014-02-28